Such a tread is generally provided with a tread pattern formed of relief elements, ridges and/or blocks, separated from one another in the transverse and/or circumferential direction by circumferential and/or transverse grooves. Depending on the axle on which a tire is fitted, its tread is provided with a pattern comprising more blocks or more ridges of generally circumferential orientation. In the case of a driving axle, i.e. an axle to which a driving or braking torque is transmitted, it is customary to produce, rather, patterns that comprise a plurality of blocks delimited by both circumferential and transverse grooves. The major disadvantages of this type of pattern are that the pliability of the tread is increased (reduction of the shear rigidity under a force tangential to the surface of the relief elements passing through the contact zone) and that the noise emission during rolling increases with the amount of grooving (ratio between the areas of grooves and the total area of the tread).
Another possibility consists in producing a pattern formed of ridges with generally circumferential orientation (i.e. which can zig-zag either side of that direction) and providing these ridges with a plurality of incisions whose width is small compared to the width of the grooves (as a general rule, the average width of the incisions is at most 2 mm); these incisions may or may not be oblique relative to the transverse direction of the tire, and may extend through part or all of the useful thickness of the tread (by definition, the useful thickness corresponds to the thickness of tread that a user can use while still complying with the legal prescriptions in force). Although the presence of these incisions favors road grip due to the presence of the numerous edges that they form on the rolling surface of the ridges, an increase of their number also reduces rigidity, which is detrimental to the tire's other performance characteristics, this reduction of rigidity being in part associated with the ability of the opposite rubber walls that delimit each incision to slide relative to one another during passage through the road contact zone.
A solution was proposed in particular by the applicant in patent EP 768958, which describes new forms of incisions that substantially reduce the relative sliding between opposite walls, and according to which said walls each have a relief surface formed of projections and concavities arranged on either side of a mean surface, a projection being completely surrounded by concavities and vice-versa, such that the wall surfaces cooperate together to restrict the relative movements between said walls.
In general these incisions are arranged so as to form rubber edges orientated essentially transversely so as to have an effect on the grip of the tire under a driving or braking torque when rolling in a straight line. It has been found that under forces tangential to the contact surface of ridges provided with such incisions, although to be sure the walls are blocked against the opposite walls, this blocking does not become effective instantaneously. During noise measurement tests on heavy vehicles that consisted in applying a driving torque to the driving axle, the applicant found that the larger was the number of incisions of the type described in patent EP 768958, the more perceptible became the noise recorded as the vehicle passed. In fact, there are directions in which there is no blocking between the walls.
The need exists for a tire tread pattern formed essentially of circumferential ridges provided with a plurality of incisions having mean width less than 2 mm, which create a large number and a large length of edges, said incisions delimiting a plurality of rubber elements whose opposite walls are mutually blocked against one another with virtually instantaneous effect (i.e. in a time which is very appreciably reduced or zero).